This invention relates generally to metal object detection and more particularly to an inductive loop structure suitable for detecting the passage or presence of a vehicle over a defined area of roadway.
Inductive loops have been used extensively for several years in various applications to facilitate detecting the presence or passage of a vehicle over a monitored area of a roadway. For example, such inductive loops have been used in conjunction with various types of electronic detectors in traffic actuated control systems for developing the input data required by a controller to control traffic signal lights. In another typical application, a loop may be connected through detector circuitry to a counter for accumulating a count of vehicle flow past a certain point. In either application, the inductive loop is intended to monitor the presence or passage of a metal mass in an area of given dimensions.
Inductive loop configurations known in the prior art have generally consisted of an insulated conductive wire (e.g. number 14 stranded copper) formed of simple rectangular geometry. A typical loop in a traffic lane might consist of one or more turns of conductor and have a width of 6 feet and a length (in the direction of traffic) of from 6 to 100 feet. Each loop operates in conjunction with electronic detector circuitry such that the loop together with components of the detector circuitry form an oscillator whose frequency is dependent upon the inductance of the loop. Loop inductance is in turn dependent on whether or not the loop is loaded by the presence of a vehicle. A vehicle over the loop decreases loop inductance and thus increases the frequency of oscillation. The remaining circuitry of the detector monitors and responds to these changes in frequency.
Although known prior art inductive loop configurations have enjoyed wide commercial acceptance (see, "Selection of Vehicle Detectors for Traffic Management," F. R. Hulscher, Traffic Engineering & Control, Dec. 1974, p. 915), they suffer from certain inherent limitations: (1) the field strength outside the loop perimeter is often sufficient to erroneously "recognize" a vehicle in an adjacent lane as being within the loop; (2) the field strength within the loop perimeter is often insufficient to recognize the presence of certain small vehicles, such as bicycles; (3) the field strength within the loop perimeter often lacks uniformity such that sensitivity in certain areas of the loop is lacking, producing "dead spots." Although any one of these problems can generally be avoided, any improvement is usually achieved at the expense of one or more of the other problems.
In view of the foregoing, the present invention is directed to an improved inductive loop configuration which substantially reduces the adverse effects of the aforementioned limitations, resulting in a more efficient monitoring of the specified area.